The present invention relates to a fluid rotating apparatus such as a vacuum pump, a compressor or the like, and a method of controlling the apparatus.
FIG. 11 shows an example of a conventional sliding vane vacuum pump provided with only one rotor. In the vacuum pump with one rotor, when a rotor 101 rotates, two blades 102 inserted into the rotor 101 in the diametrical direction of the rotor 101 are driven and rotated inside a cylindrical fixed wall 103 (stator). At this time, the leading ends of the blades 102 are kept in contact with the fixed wall since the blades 102 are always urged in the radial direction of the rotor 101 by the action of a spring 104. Subsequent to the rotation, the capacity of each of the spaces 105 partitioned by the blades 102 in the fixed wall is changed, and a gas entering from a suction port 106 formed in the fixed wall is eventually sucked and compressed and flows out through a discharge port 107 having a discharge valve. In the vacuum pump of this type, in order to prevent internal leakage, it is necessary to seal the side surface and the leading ends of the blades 102, the side surface of the fixed wall 103, and the side surface of the rotor 101 with an oil membrane, respectively. However, when this kind of vacuum pump is used in the manufacturing process of semiconductors, e.g., CVD or dry etching, etc. using a highly corrosive reactive gas such as chlorine gas, the gas reacts with the sealing oil to thereby generate a reaction product in the pump. In such case as above, it becomes necessary to perform maintenance work frequently so as to remove the reaction product, and moreover the pump should be cleaned and the oil should be exchanged every time maintenance work is performed, thus bringing the manufacturing process to a halt. The activity rate is hence decreased. So long as the sealing oil is used in the vacuum pump, the oil is scattered from the downstream side to the upstream side, polluting the vacuum chamber and reducing the manufacturing efficiency.
In view of the above-described inconveniences, a positive displacement screw vacuum pump has been developed and put into practical use as a dry pump not requiring the sealing oil. FIG. 12 is a side sectional view of an example of the screw vacuum pump. Within a housing 111 are provided two rotors 112, the rotary shafts of which are made parallel to each other. Spiral grooves are formed on the peripheral surfaces of the rotors 112. A space is defined when a recess portion (groove) 113a of one rotor and a projection 113b of the other rotor are meshed with each other. As the rotors 112 rotate, the capacity of the space is changed, thereby causing sucking/discharging of the fluid.
In the conventional positive displacement screw vacuum pump referred to above, the synchronous rotation of the rotors 112 is achieved by timing gears. That is, the rotation of a motor 115 is transmitted from a driving gear 116a to an intermediate gear 116b and further to one of the meshed timing gears 116c of the rotors 112. The phase of the rotating angles of both rotors 112 is adjusted by the engagement between the two timing gears 116c. Therefore, since the screw vacuum pump uses the gears both for transmission of the motor power and for synchronous rotation of the rotors as described hereinabove, a lubricating oil filled in a machine chamber 117 which houses the gears must be supplied to the gears. Moreover, a mechanical seal 119 should be provided between the machine chamber 117 and a fluid chamber 118 so as to prevent the lubricating oil from entering the chamber 118 where the rotors are accommodated.
The vacuum pump with two rotors in the above-described construction has such disadvantages yet to be solved that (1) many gears are required for the power transmission and the synchronous rotation, i.e., many parts are required, resulting in a complicated structure of the apparatus (2) a high speed operation cannot be expected and the apparatus becomes bulky in size since the rotors are synchronously rotated with contact between the gears, (3) a mechanical seal must be regularly exchanged due to the abrasion thereof, failing to realize a completely maintenance-free pump, (4) a large sliding torque due to the mechanical seal induces a great mechanical loss, and so on.